Cyclists are facing more of a mental challenge than a physical one, organisers say, as they race a gruelling 5,500 kilometres from Fremantle to Sydney.

About 70 riders set off last Saturday and the leaders of the Indian Pacific Wheel Race have now crossed into South Australia, in what organiser Jesse Carlsson says is an "unsupported event" for the participants.

"Riders are completely unsupported, so they're just like any bike tourer that you'd see out on there on the road travelling across Australia," he said.

"They've got to carry all the stuff they need to survive out there — sleeping equipment — they need to carry all their food and water.

"It's not like the Tour de France or any of those supported cycling races — the riders are out there alone."

Cyclists from more than a dozen countries are taking part, crossing the Nullarbor then heading for Adelaide, along the Great Ocean Road in Victoria and then north over the Great Dividing Range until they reach the Sydney Opera House.

"The route doesn't take the most direct course across Australia," Mr Carlsson said.

"One of the things we'd really like to do with this event is have an established bike touring route across the country that's a bit more interesting, perhaps, than the most direct route across."

The race leaders hope to make it to Sydney in 12 days, with Belgian Kristof Allegaert heading the field, and British cyclist Mike Hall currently second.

Mr Carlsson said mental stamina can count for more than physical abilities.

"The riders often say that the riding's almost the easy part. It's largely a mental battle, which I think surprises people [on] these challenges," he said.

"Even former pros have taken on these challenges and they find that they really struggle after about three days or so, just coping with the boredom, monotony of long straight roads that just don't end."